basic/process-util: allow quoting of commandlines

Since the new functionality is controlled by an option, this causes no change
in output yet, except tests.

The login in the old branch of !(flags & PROCESS_CMDLINE_QUOTE) is essentially
unmodified. But there is an important difference in behaviour: instead of
unconditionally reading the whole virtual file, we now read only 'max_columns'
bytes. This makes out code to write process lists quite a bit more efficient
when there are processes with long command lines.

src/basic/process-util.h

@@ -35,6 +35,7 @@
 typedef enum ProcessCmdlineFlags {
         PROCESS_CMDLINE_COMM_FALLBACK = 1 << 0,
         PROCESS_CMDLINE_USE_LOCALE    = 1 << 1,
+        PROCESS_CMDLINE_QUOTE         = 1 << 2,
 } ProcessCmdlineFlags;
 
 int get_process_comm(pid_t pid, char **name);